In a square rod (rectangular rod or slab) formed in the form of a square pillar, which is used as a laser medium for a solid state laser apparatus, it is easy to install a mechanism (metallic heat sink or the like) for dissipating heat generated in the square rod from a couple of lateral surfaces facing each other which serve as heat sinking surfaces and it is therefore easy to dissipate the heat from the square rod, because the heat sinking surfaces are planar.
Furthermore, a square rod has a feature of easily providing laser oscillation of linear polarization by causing birefringence to occur in the square rod in one direction because a temperature gradient is produced only in a direction of heat sinking if ideal heat sinking is carried out. Therefore, a pumping module that uses a square rod is suitable for spaceborne laser equipment which requires conduction cooling, laser equipment intended for laser machining which requires high average laser power, and so on.
FIG. 1 is a diagram showing the structure of a prior art pumping module which is so constructed as to use a square rod. For example, the pumping module of FIG. 1 is disclosed in pp. 434 of the following reference 1.
<Reference 1>
Springer Series in Optical Sciences Vol.1 “Solid-State Laser Engineering the Fourth Version”, written by Walter Koechner and printed by Germany Springer Co. in 1996
In FIG. 1, reference numeral 1 denotes a square rod, reference numeral 1a denotes a heat sinking surface of the square rod 1, reference numeral 1b denotes another heat sinking surface which is perpendicular to the heat sinking surface 1a of the square rod 1, reference numeral 2 denotes cooling water, reference numeral 3 denotes a pump light source, and reference numeral 4 denotes an optical axis. In FIG. 1, y axis coincides with a direction of heat sinking, z axis coincides with the optical axis 4, and x axis coincides with a direction perpendicular to the y axis and the z axis.
Next, a description will be made as to the operation of the prior art pumping module.
In the pumping module that uses the square rod 1 of FIG. 1, pump light emitted out of the pump light source 3 is absorbed by the square rod 1, and this results in generation of a gain. The pump light thus amplifies laser light that propagates in the direction of the optical axis. Heat generated in the square rod 1 is dissipated via the heat sinking surface 1a in the direction of the y axis by the cooling water 2.
In the prior art pumping module, because a temperature gradient is produced only in the direction of heat sinking when generated heat is ideally dissipated from the heat sinking surface 1a, the two axes (fast axis and slow axis) of heat birefringence caused by the temperature gradient appear in the direction of the y axis and in the direction of the x axis, respectively. Therefore, when laser light linearly polarized in the direction of the y axis or x axis is incident upon the pumping module of FIG. 1, the laser light can propagate within the square rod 1 with the linear polarization being held, and therefore the loss in the cavity due to decrease in the extinction ratio can be reduced and the laser oscillation of linear polarization can be facilitated.
However, the pumping module of FIG. 1 has the following drawbacks.
In other words, because a thermal lens effect according to the temperature gradient is produced only in the direction of heat sinking (the direction of the y axis), and no thermal lens effect is produced in the direction (the direction of the x axis) perpendicular to the direction of heat sinking, the square rod 1 serves as a cylindrical lens that provides a lens effect only in the direction of the y axis, and that provides astigmatism for the laser light passing therethrough. Therefore, a problem encountered in the pumping module is that a mechanism of compensating for astigmatism is needed when the pumping module of FIG. 1 is used for such a laser apparatus as a laser oscillator or a laser amplifier, just as it is, and the optical system structure thus becomes complex.
The structure of a cavity that uses a pumping module having a modified square rod is disclosed, as a technique for solving this problem, in pp. 437 of the above-mentioned reference 1.
FIG. 2 is a diagram showing the structure of a laser oscillator to which the prior art pumping module is applied. The same reference numerals as shown in FIG. 1 denote the same components or like components.
In FIG. 2, reference numeral 5 denotes a square rod whose both ends are ground so that they have a Brewster Angle, reference numeral 5a denotes a heat sinking surface of the square rod 5, reference numeral 5a denotes a lateral surface perpendicular to the heat sinking surface 5a of the square rod 5, reference numeral 6 denotes a total reflection mirror, reference numeral 7 denotes a partial reflection mirror, and reference numeral 8 denotes an optical path along which laser light propagates within the laser oscillator.
Next, a description will be made as to the operation of the prior art laser oscillator.
In the laser oscillator of FIG. 2, because laser light propagates along a zig-zag optical path in the square rod 5, temperature gradients caused in the direction of heat sinking are made uniform and therefore the thermal lens effect can be compensated for. Furthermore, because no thermal lens effect is produced in the direction of the x axis when heat is ideally dissipated from the heat sinking surface 5a, the astigmatism can be compensated for.
However, because the square rod 5 actually used has a limited size in the case of using the pumping module shown in FIG. 2, heat is also dissipated from the lateral surface 5b through radiation and conduction even though heat is dissipated from the heat sinking surface 5a. Thus a problem encountered in the prior art laser oscillator is that it is difficult for heat to be ideally dissipated only in the direction of the y axis, and the direction of the temperature gradients is not parallel to the direction of the y axis and therefore there causes an inclination in the direction of the temperature gradients.
This problem will be explained a little more in detail.
FIGS. 3(a) and 3(b) are diagrams for explaining an inclination of the directions of the temperature gradients in the square rod 5 of the laser oscillator of FIG. 2, and each of them shows a cross section of the square rod 5 taken along a plane perpendicular to the z axis. FIG. 3(a) shows a case where heat is ideally dissipated from the heat sinking surface 5a, and FIG. 3(b) shows a case where there causes variations in the directions of the temperature gradients in the square rod 5 having a limited size. The same reference numerals as shown in FIG. 2 denote the same components and like components.
In the case of FIG. 3(a), because heat is ideally dissipated only from the heat sinking surface 5a, all the directions of the temperature gradients become parallel to the y axis and the birefringence axes appear in the direction of y axis and in the direction of x axis, respectively, wherever in the cross section of the square rod 5. Therefore, when laser light linearly polarized in the direction of the y axis or x axis is incident upon the square rod 5, the laser light propagates within the square rod 5 with the linear polarization thereof being held. Because the temperature gradients are produced only in the direction of the y axis, a thermal lens effect is produced only in the direction of the y axis and no thermal lens effect is produced in the direction of the x axis.
On the other hand, even in the case of FIG. 3(b), when focused to the center of the square rod 5 and line segments on A axis and B axis of the square rod 5, the directions of the temperature gradients are parallel to the y axis and the birefringence axes appear in the direction of the y axis and in the direction of the x axis, respectively.
However, ununiformity is caused in the directions of the temperature gradients produced in the cross section of the square rod 5 because the square rod 5 has a limited size and therefore heat is also dissipated from the lateral surface 5b through radiation and conduction. In other words, when focused to other than the center of the square rod 5 and line segments on the A axis and B axis of the square rod 5, the temperature gradients are inclined against the direction of the y axis.
Therefore, there cause variations in the orientation of the two birefringence axes in cross section of the square rod 5. When laser light linearly polarized in the direction of the y axis or the x axis passes through the square rod 5 of FIG. 3(b), a decrease occurs in the degree of linear polarization and hence a decrease occurs in the extinction ratio due to the ununiformity of the birefringence. Furthermore, when the pumping module of FIG. 2 is applied to such a laser apparatus as a laser oscillator or a laser amplifier, a decrease in the efficiency of energy and a decrease in the beam quality can occur.
Moreover, temperature gradients are produced in the direction of the x axis and a weak thermal lens effect is produced because the directions of the temperature gradients have an inclination with respect to the y axis. This means that astigmatism occurs due to the thermal lens effect produced in the direction of the x axis while the thermal lens effect in the direction of the y axis can be canceled in the prior art pumping module as shown in FIG. 2. Therefore, a problem with the prior art pumping module is that a mechanism of compensating for astigmatism is needed for such a laser apparatus to which the pumping module of FIG. 2 is applied as a laser oscillator or a laser amplifier, and the optical system structure thus becomes complex.
A problem with prior art pumping modules constructed as mentioned above is that the extinction ratio of laser light passing through the square rod is decreased because it is difficult to provide ideal heat sinking such that temperature gradients are produced only in the direction of the y axis and this results in occurrence of a variation in the orientation of the birefringence axes.
Another problem with prior art pumping modules is that astigmatism occurs because a thermal lens effect is also produced in the direction of the x axis.
Another problem encountered in such a laser apparatus to which a prior art pumping module is applied as a laser oscillator or a laser amplifier is that a decrease in the efficiency of energy and a decrease in the beam quality can occur, and a mechanism of compensating for astigmatism is needed and the optical system structure thus becomes complex.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a pumping module capable of preventing any decrease in the extinction ratio which is caused by a variation in the orientation of the birefringence axes that occurs in a square rod, and reducing astigmatism.
It is another object of the present invention to provide a laser oscillator and a laser amplifier capable of preventing any decrease in the efficiency of energy and any decrease in the beam quality without having to use a mechanism for compensating for astigmatism.